[Vm-dev] [Fwd: Image file loading]

[Andreas Raab]

David T. Lewis wrote:
> On Tue, Jan 19, 2010 at 02:30:43PM -0800, Andreas Raab wrote:
>> What I'm proposing is an additional VM entry point, called
>>
>>   Interpreter>>readImageFromHeap: heapStart size: heapSize
>>
>> which takes to arguments: The pointer to the beginning of the
>> (pre-allocated) memory portion where the image file has been loaded or
>> mapped and a size for the allocated memory portion. The VM then assumes
>> that beginning at heapStart there is an image provided that it needs to
>> properly prepare (i.e., perform byte-reversal, pointer adjustment etc).
> 
> A suggestion:
> 
>   Interpreter>>installImage: address size: imageSize header: header

I don't like that. It implies that the support code needs to be able to 
dissect the image file to find out how much is header and how much is 
data. That implies the support code needs to know about image format, 
byte reversal, and several other things. The interpreter already does 
that so why should we have to duplicate that in the support code again?

> Rationale:
> 
> - "Install" is more appropriate that "read", since the contents of the
>   image file have already been read.
> 
> - "Heap" is not relevant.

Fair enough. I don't really care what it's called.

> - The file header is logically distinct from from the contents of the
>   image proper, and since someone else did the reading, it seems better
>   to pass them as two distinct things (*). The pointer to header is
>   presumed to be a memory block of at least 64 bytes, so in the usual
>   case of a 32 bit image read into a block of malloc'ed space, we expect
>   that address == header + 64.

See above. You're making it artificially harder for the support code by 
requiring it to dissect the file and figure out what the header is. 
There is really no point to that.

> My opinion: Adding the entry point is easy to do and has a clear benefit
> for at least one platform of interest, so we should do this. Removing
> the existing entry point is debatable and can be done at a later time.

Why do you think removing the existing entry point is debatable? I can't 
see a long-term benefit for using readImageFromFile: given that the new 
entry point trivially subsumes the old one.

Cheers,
   - Andreas